If your using 2007 you can use the replace resource wizard in Proj Pro.
Tools/Build Team From Enterprise, then replace the new resource for the old
one.
Actuals for the old resource will be maintained and remaining work will be
assigned to the new resource.
The down side is that you will have to do this for each plan where the old
resource has an assignment.
In 2003, I seem to remember there was an global admin function to replace
resources.